The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
For corrosion to occur, there must normally be at least two dissimilar metals, an electrolyte (water with any type of salt dissolved in it, for example), and a path between the dissimilar metals that serves as a conductor. Cathodic protection is a widely-used technology employed to protect and control the corrosion of metallic structures, such as pipelines, wells, piers, buildings, storage tanks, ships, off-shore oil platforms, on-shore oil well casings, and other metal structures that are buried or submerged in corrosive electrolytes. Due to its wide use, cathodic protection has become a requirement and/or best practice for controlling the corrosion of various structural metallic components immersed in soil or water.
Cathodic protection prevents corrosion by converting all of the anodic (active) sites on a metal surface to cathodic (passive) sites by supplying electrical current from an alternate source. An anode discharges electrical current according to Ohm's law, which is I=E/R, where I is current flow, E is the difference in potential between the anode and the cathode, and R is the total circuit resistance. Initially, because the difference in potential between the anode and the cathode is high, current will be high. However, as the difference in potential decreases (due to the effect of the current flow on the cathode and the polarization of the cathode), the current will gradually decrease. Generally, the length of the anode is used to determine how much current the anode can produce, and thus, how much surface area can be protected, and the weight of the anode is used to determine the period of time for which the anode can sustain a proper level of protection.
Cathodic protection can be accomplished using sacrificial anodes or impressed current. In a sacrificial anode system, cathodic protection is achieved first by using an alternate source, such as an easily-corrodible, highly-active metal; then by making the alternate source the cathode of an electrochemical cell—the electrode through which electric current flows out of the polarized electrical device; and lastly by placing the alternate source in contact with a less-active metal that is to be protected. The easily-corrodible metal acts as the anode of the electrochemical cell—the electrode through which positive electric charge flows into the polarized electrical device. Because galvanic anodes sacrifice themselves to protect the metal surface that is desired to be protected, this technique is referred to as a sacrificial cathode system.
For larger structures, sacrificial anode systems are not likely to be used, as the sacrificial anodes cannot economically deliver enough current to provide complete protection. However, impressed current cathodic protection systems can be effective for larger structures because those systems use anodes connected to a direct current power source (a cathodic protection rectifier), and as in sacrificial anode systems, the impressed current systems depend on a supply of high energy electrons to stifle anodic reactions on the metal surface. Further, in the impressed current system, the high energy electrons are supplied by the rectifier, such that low energy electrons picked up at a non-reactive anode bed are given additional energy by the action of the rectifier to be more energetic than the electrons that would be produced in the corrosion reaction.